Problem: If $\angle QPS$ is a right angle and $m \angle RPS = 81^\circ$, what is $m \angle QPR$, in degrees? $81^\circ$ $P$ $Q$ $S$ $R$
Explanation: From the diagram, we see that $\angle QPR$ and $\angle RPS$ are complementary angles. Therefore, $m \angle QPR + m \angle RPS = 90^\circ$ Thus, $m \angle QPR = 90^\circ - m \angle RPS = 90^\circ - 81^\circ = 9^\circ$.